The present invention relates to a color cathode ray tube, and particularly to a color cathode ray tube having a three beam in-line, dynamic focus type electron gun capable of providing good focus characteristics over the entire screen area and good display contrast with a reduced dynamic focus voltage for its electrostatic quadrupole lens.
Color cathode ray tubes having an in-line type electron gun for use in TV receivers or display monitors have a phosphor screen formed on the inner surface of a faceplate of its panel portion, a shadow mask closely spaced from the phosphor screen within the panel portion, a deflection yoke mounted around its funnel portion, and an in-line type electron gun housed in its neck portion. The in-line type electron gun includes three cathodes arranged in line, and at least the first grid (G1) electrode, the second grid (G2) electrode, the third grid (G3) electrode and an anode, and projects three electron beams toward the phosphor screen.
To obtain good display image at the periphery of the phosphor screen as well as the center of the phosphor screen, that is, uniform resolution over the entire phosphor screen by using a color cathode ray tube having an in-line type electron gun, it is known to employ an electron gun of the dynamic focus type in which an electrostatic quadrupole lens is formed between two adjacent ones among electrodes of the in-line type electron gun and one of the two is supplied with a fixed focus voltage and the other of the two is supplied with a fixed focus voltage superposed with a dynamic voltage varying with deflection of the electron beams.
FIG. 4 is a cross-sectional view of a prior color cathode ray tube employing an in-line type electron gun of the dynamic focus type (hereinafter referred to as a DF type in-line electron gun).
In FIG. 4, reference numeral 41 denotes a panel portion, 41F is a faceplate, 42 is a neck portion, 43 is a funnel portion, 44 is a phosphor screen, 45 is a shadow mask, 46 is an internal conductive coating, 47 is a DF type in-line electron gun, 48 is a deflection yoke.
A grid electrode occupying the nth position counting from a cathode is called a grid n electrode in this specification.
A grid occupying the nth position counting from a cathode is called a Gn in this specification.
In the DF type in-line electron gun 47, reference numerals 50.sub.1, 50.sub.2 and 50.sub.3 denote cathodes, 51 is a G1 electrode, 52 is a G2 electrode, 53 is a G3 electrode, 54 is a G4 electrode, 55(1) is a first G5 sub-electrode, 55(2) is a second G5 sub-electrode, 56 is a G6 electrode (an anode), 57 is a shield cup, 58 are vertical electrode pieces, and 59 are horizontal electrode pieces.
The glass bulb of the color cathode ray tube comprises a panel portion 41, a neck portion 42 and a funnel portion 43. The panel portion 41 is provided with the phosphor screen 44 coated on the inner surface of its faceplate 41F and the shadow mask 45 closely spaced from the phosphor screen 44 within the panel portion 41. The funnel portion 43 is provided with the internal conductive coating 46 in its inner surface and the deflection yoke 48 mounted on its the outer surface. The neck portion 42 houses the DF type in-line electron gun 47 therein.
The DF type in-line electron gun 47 comprises three cathodes 50.sub.1, 50.sub.2 and 50.sub.3 arranged in line in a horizontal plane, and following the cathodes, the G1 electrode 51, the G2 electrode 52, the G3 electrode 53, the G4 electrode 54, the first G5 sub-electrode 55(1), the second G5 sub-electrode 55(2), the G6 electrode 56, the shield cup 57, arranged along the axis of the cathode ray tube in the order named. One center and two side electron beam apertures in each of the G1 electrode 51, the G2 electrode 52, the G3 electrode 53, the G4 electrode 54, the first G5 sub-electrode 55(1), the second G5 sub-electrode 55(2), the G6 electrode 56, and the shield cup 57 are aligned with center lines O.sub.2, O.sub.1 and O.sub.3 of the cathodes 50.sub.2, 50.sub.1, and 50.sub.3, respectively.
In the G6 electrode 56, the center line of the center electron beam aperture is aligned with the center line O.sub.2 of the corresponding cathode 50.sub.2, and the respective center lines of the two side electron beam apertures are slightly displaced outwardly with respect to the center lines O.sub.1 and O.sub.3 of the corresponding cathodes 50.sub.1 and 50.sub.3, respectively. The first G5 sub-electrode 55(1) is provided with the vertical electrode pieces 58 sandwiching horizontally each of the three electron beam apertures in its end facing the second G5 sub-electrode 55(2), and the second G5 sub-electrode 55(2) is provided with a pair of the horizontal electrode pieces 59 sandwiching vertically the three electron beam apertures in common in its end facing the first G5 sub-electrode 55(1). The vertical electrode pieces 58 and the horizontal electrode pieces 59 form an electrostatic quadrupole lens between the first and second G5 sub-electrodes 55(1), 55(2).
In operation, the first G5 sub-electrode 55(1) is supplied with a fixed focus voltage, the second G5 sub-electrode 55(2) is supplied with a fixed focus voltage superposed with a dynamic voltage varying with deflection of the electron beams, and the G6 electrode 56 serving as an anode, the shield cup 57 and the internal conductive coating 46 are supplied with an accelerating voltage (an anode voltage).
In the prior art color cathode ray tube, three electron beams emitted from the three cathodes 50.sub.1, 50.sub.2, 50.sub.3 of the DF type in-line electron gun 47 travel accelerated and focused along the respective center lines O.sub.1, O.sub.2, O.sub.3 through the electron beam apertures in each of the G1 electrode 51, the G2 electrode 52, the G3 electrode 53, the G4 grid electrode 54, the first G5 sub-electrode 55(1), the second G5 sub-electrode 55(2), the G6 electrode 56, the shield cup 57, and are projected from the electron gun 47 toward the phosphor screen 44. The three electron beams projected from the electron gun 47 are properly deflected horizontally and vertically by the deflection yoke 48, then pass through an electron beam aperture in the shadow mask 45 and impinge upon the phosphor screen 44 to produce a desired image on the phosphor screen 44.
Color cathode ray tubes for use in color display monitors and the like usually employ a self-converging deflection yoke 48 of the type having both horizontal and vertical deflection windings wound in a saddle configuration (hereinafter referred to as the saddle/saddle type) to prevent magnetic fields generated by the deflection yoke 48 from radiating from the monitor to its outside.
The self-converging deflection yoke 48 increases deflection defocusing on the phosphor screen 44 due to the inherent non-uniformity in its deflection magnetic fields, deteriorates image resolution at the periphery of the phosphor screen 44 and therefore an electrostatic quadrupole lens is employed in the in-line type electron gun 47 with a dynamic focus voltage varying with deflection of the electron beams.
When the deflection of the electron beams is zero or very small, that is, when the electron beams scan the central portion of the phosphor screen 44, a dynamic voltage becomes zero or very small, a focus voltage applied to the first G5 sub-electrode 55(1) becomes equal or nearly equal to a focus voltage applied to the second G5 sub-electrode 55(2), the strength of the electrostatic quadrupole lens is weakened and consequently no astigmatism is produced in the electron beam spot at the center of the phosphor screen 44.
When the deflection of the electron beams is large, that is, when the electron beams scan the periphery of the phosphor screen 44, the dynamic voltage becomes large, the focus voltage applied to the second G5 sub-electrode 55(2) becomes higher than the focus voltage applied to the first G5 sub-electrode 55(1) and the strength of the electrostatic quadrupole lens becomes stronger to produce astigmatism of the electron beams deflected to the periphery of the phosphor screen 44. This astigmatism causes the shape of the beam spot on the phosphor screen to elongate its core portion vertically and to elongate its halo horizontally such that deflection defocusing caused by the self-converging deflection yoke 48 is canceled out and resolution at the periphery of the phosphor screen 44 is improved.
In a color cathode ray tube employing the prior art DF type in-line electron gun, a distance between its main lens and the periphery of the phosphor screen 44 is longer than that between its main lens and the center of the phosphor screen 44, and the electron beam focusing condition for the center of the phosphor screen 44 differs from that for the periphery of the phosphor screen 44 such that adjustment for the best beam focus at the center of the phosphor screen 44 degrades the beam focus and resolution at the periphery of the phosphor screen 44. If a correction lens for curvature of the image field is incorporated in the DF type in-line electron gun 47, when the electron beams are deflected to the periphery of the phosphor screen 44, a focus voltage applied to the second G5 sub-electrode 55(2) becomes higher, a difference between the focus voltage and an accelerating voltage (an anode voltage) applied to the G6 electrode 56 decreases and the strength of the focus lens weakens such that the focus point (the image point) of the electron beams is moved toward the phosphor screen 44, the electron beams deflected to the periphery of the screen 44 are focused on the phosphor screen 44 and deterioration in resolution at the periphery of the screen 44 is prevented. In this way, by using a dynamic voltage, the prior color cathode ray tube can correct curvature of the image field as well as astigmatism in electron beam spots.
The prior art color cathode ray tube corrects astigmatism in beam spot and curvature of the image field by applying a dynamic voltage to the second G5 sub-electrode 55(2) of an electrostatic quadrupole lens. If a color cathode ray tube for use in a color monitor or the like employs a deflection yoke 48, of a relatively wide deflection angle, 95.degree. to 105.degree., for example, to reduce the depth of the monitor, a required dynamic voltage becomes a little too high for a color monitor due to its large deflection angle of the electron beams, and a distance between the main lens and the phosphor screen (hereinafter referred to as a lens-screen distance) becomes shorter such that the scanning electron beams and electron beam apertures in the shadow mask 45 interfere with each other and produce raster moire (horizontal spurious stripes) on the phosphor screen.
To solve the above problems in the DF type in-line electron gun, the present inventors previously proposed an electron gun satisfying the following inequalities to reduce the magnitude of a dynamic voltage and reduce appearance of raster moire (horizontal spurious stripes) on the phosphor screen: EQU 0.06.times.L(mm).ltoreq.B-20.times.A/(3.phi.).ltoreq.19.0(mm),
and EQU L.ltoreq.352(mm)
where
A(mm) is an axial length of the G4 electrode, PA1 .phi.(mm) is a diameter of an aperture in the G4 electrode, PA1 B(mm) is an axial length of the G5 electrode, and PA1 L(mm) is a distance between the end of the G5 electrode on its phosphor side and the phosphor screen.
When the proposed color cathode ray tube employs a dark tainted panel (light transmission of 38%, for example) for a faceplate of a panel portion to increase its display contrast ratio and it is operated to provide the display brightness equal to that of a color cathode ray tube employing a tainted panel (light transmission of 50%, for example), there arises a new problem that electron beam spots on the phosphor screen are enlarged.
For example, if the proposed color cathode ray tube employs a faceplate with its light transmission reduced by about 20% compared with that of a tainted panel by using a dark-tainted panel and by applying antistatic and antireflection coating on the dark-tainted panel if necessary, a beam current for each cathode has to be increased by about 30% to obtain a brightness equivalent to that of a color cathode ray tube employing the tainted panel and consequently its beam spot diameter is increased by about 10%.